1. Field of the Invention
This invention relates to a phosphor which emits green light by irradiation of ultraviolet rays produced by a low pressure mercury vapor discharge lamp. More particularly, it is concerned with such a phosphor which is excellent in its temperature-versus-luminance characteristics.
2. Discussion of Background
In recent years, there has been made much use of the so-called "3-band light emitting type fluorescent lamp" having high operating efficiency and high color-rendering property in comparison with white fluorescent lamp for general lighting purpose. This type of fluorescent lamp utilizes a mixture of three kinds of phosphors which emit lights in blue, green and red, each having a narrow band of the light emission. Hence, efforts are being made for each and every phosphor to improve its light emitting efficiency, to optimize its light emitting spectrum, and, so forth. Since the green-emitting phosphor has the main wavelength of its light emitting spectrum in the region of the highest luminous efficiency, the light emitting efficiency of it greatly influences on the brightness of the lamp. For this reason, studies and researches have been conducted in various aspects concerning improvement in the properties of the (La, Ce, Tb)PO.sub.4 phosphor which is one of the green-emitting phosphors for the 3-band fluorescent lamp. According to Japanese Unexamined Patent Publication No. 56086/1979, for example, the emission intensity is said to increase by sub-stitution of gadolinium (Gd) and yttrium (Y) for a part of lanthanum (La). Also, Japanese Unexamined Patent Publication No. 23674/1982 discloses that the emission intensity of the (La, Ce, Tb)PO.sub.4 phosphor reaches its maximum with a substituting quantity of cerium (Ce) in a range of from 0.6 to 0.8 mole.
However, such phosphors have in general their optimum condition for the emission intensity in the vicinity of room temperature, but no consideration has been given on their temperature characteristics.
In recent years, small-sized fluorescent lamps are being widely used in place of incandescent lamps. Such small-sized fluorescent lamps have a high load on their tube wall in comparison with those conventional straight or circular tube type fluorescent lamps. Thus, the temperature of the tube wall of these small-sized fluorescent lamps reaches as high as 100.degree. C. or above. Accordingly, the temperature of the phosphor layer coated on the inner surface of the tube wall becomes much higher. It has been well known that, in the afore-mentioned (La, Ce, Tb)PO.sub.4 green-emitting phosphor, when cerium (Ce) is in a high concentration, the temperature-versus-luminance characteristics of it becomes considerably deteriorated. Even if the concentration of Ce is reduced, there occurs extinction of light at 150.degree. C. or above.
A curve (II) in the graphical representation of FIG. 1 of the accompanying drawing shows the temperature-versus-luminance characteristics of the phosphor composed of (La.sub.0.62 Ce.sub.0.20 Tb.sub.0.18)PO.sub.4, wherein the light extinction is seen to have taken place at a temperature of 150.degree. C. and above. Accordingly, when the above-mentioned conventional (La, Ce, Tb)PO4 phosphor is used as the green component for the small-sized fluorescent lamp, the lamp reduces its luminance or discolors as the temperature on the tube wall rises with passage of time after lighting.
With a view to solving the various problems as mentioned above, the present inventors have conducted extensive studies and researches, as the result of which they have found a green-emitting phosphor which is excellent in its temperature-versus-luminance characteristics. It brings about no decrease in the emission intensity at the room temperature, and no extinction of light even in the temperature region of 350.degree. C.